Method of manufacture of insulation for coaxial cables



June 10, 1952 c, MILDNER 2,599,857

METHOD OF MANUFACTURE OF INSULATION FOR COAXIAL CABLES V I5 Sheets-Sheet 1 Filed Jan. 15, 1947 June 10, 1952 c, MlLDNER 2,599,857

METHOD OF MANUFACTURE OF INSULATION FOR COAXIAL CABLES Filed Jan. 13, 1947 5 Sheets-Sheet 2 W42 fiC M METHOD OF MANUFACTURE OF INSULATION FOR COAXIAL CABLES Filed Jan. 13, 1947 5 Sheets-Sheet 5 g ,1 a i h T m &

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Patented June 10, 1952 METHOD OF MANUFACTURE OF INSULA- TION FOR COAXIAL CABLES Raymond Charles Mildner, Oxhey, England, as

signor to Telegraph Construction and Maintenance Company Limited, London, England, a company of Great Britain Application January 13, 1947, Serial No. 721,843 In Great Britain January 18, 1946 4 Claims. 1

This invention relates to improvements in air space electric cables.

In the design of an air space electric cable for the transmission of high-frequency power, it is desirable that the efiective permittivity of the cable insulant should be made as low as possible in order to achieve a high efficiency of transmission. In practice this means that as little solid insulation material as possible should be used, particularly at the parts which are subjected to the highest electrical stresses, that is, near the inner conductor or conductors. It is also important that the insulation should be uniform throughout the length of the cable, so as to reduce the electrical reflections from points of discontinuity to a minimum. Such discontinuities might be caused by a change in the effective permittivity of the insulation at points along the cable length, or by changes in the relative dimensions of the inner and outer conductors.

A further principle of design lies in the requirement that the inner conductor or conductors should be mechanically coupled, both closely and uniformly along the length, to the outer conductor, so as to minimis relative movement of the conductors by reason of handling or differential thermal expansion. In practice these various requirements are conflicting, and practical designs attempt to achieve a compromise.

One known method of constructing a cable, which partly achieves the desirable characteristics noted above, is as follows: The inner conductor is lapped with a circular thread of insulatmg material of about 1 mm. diameter and a pitch of about 2 cm. This assembly is then provided with an extruded outer sleeve of flexible plastic material, which serves the purposes of locking the thread assembly, giving the cable construction a measure of robustness and providing a watertight covering. 7

The limitations of this design are due to the maximum size of thread which can be used, the need for avoiding an exceptionally long lay to the thread and the impossibility of locking the thread securely to the inner conductor, because the inner conductor tends to be distorted by th lateral pressure of the thread. In practice, an efiective permittivity of about 1.4 with insulating material of permittivity 2.3 is as low as can be achieved.

It has been realised that the use of thread of rectangular cross-section, in which one narrow side is laid along the inner conductor of the cable and the wide sides lie radially, would provide a more advantageous construction than that which incorporates a circular thread.

It has been proposed, for instance, that such helical threads should be formed by extrusion around a central conductor by means of a rotating die. In practice, however, it is extremely diflicult to form such a thread with the required shape and uniformity owing to the sensitivity of the process to minor fluctuations in the pressure and temperature, which may occur within the extrusion machine.

It has also been proposed, in a transmission system having concentric conductors in which the outer conductor is supported by a spiral strip-of insulation material with its inner edge coiled around the inner conductor, to form the helical strip by shaving a so-called spiral ribbon from the edge of a hollow cylindrical mass of hard rubber of such dimensions that, when stretched out, the ribbon will assume the desired helical shape. In practice it is difiicult to form a hollow cylindrical mass of insulating material in the requisite lengths with a uniformity of inner and outer diameters sufficiently great for a satisfactory construction.

The present invention has for its object a method of producing, in helicalform, a membrane-lik thread which can be used in the construction of an air space cable having greatly improved characteristics, both electrical and mechanical, over other constructions hitherto proposed. Furthermore thereare no practical limitations to the length of cable which can be produced in this way. l

According to the present invention helical strip insulation material for use in the manufacture of an electric cable of the air space type is obtained by the application to a former of predetermined size of a layer of dielectric material, cutting the layer into a short pitch helix, and then separating the helix so formed.

In carrying out the invention a former or support which is of the same or substantially the same diameter as the central conductor or coil of the cable to be constructed has applied, such as by extrusion, a layer of dielectric material, and from this layer is obtained by cutting a shortpitch helix, which, after removal from th former. and the necessary expansion,is preferably subjected to heat treatment to remove any stress set up by the expansion from a short pitch to an open pitch helix.

With this method it is possible accurately to control the dimension of the resultant helix since the former prevents any deformation of the insulation layer during the cutting operation and the dimensions of both the inner and outer diam- 3 eter of the strip can be uniformly varied throughout the length of the helix to suit the design of cable for which it is required.

As applied to a cable having coaxial conductors, the helical membrane may be directly mounted on the inner conductor or applied over a layer of insulation previously applied to the conductor.

For a large size cable, two helical membranes may be used, the innermost membrane being locked in position on the central conductor by an extruded layer of insulation and over which the outer membrane is applied.

The outer conductor is applied direct to the dielectric membrane or over an extruded sleeve of insulation. In the case of a cable having twin or multiple conductors, thehelical membrane is used to locate the conductors and for this purpose is preferably provided with edge slots to accommodate each conductor, the latter being locked in their relative positions by securin means, which may conveniently consist of a second helical-membrane.

The invention is illustrated-in the accompanying drawings in which Figure 1 is-an elevation showing the former or support with a layer of dielectric and the method of cutting a helical membrane thereon.

Figure 2 is an elevation showing a short length of helical membrane after removal of the support and after it has been extended to' increase the pitch and as in the final cable-assembly.

Figures 3, 4 and 5am end views of the helical membrane with different formsof locating slots for'use'in the manufacture of a multioonductc-r cable.

Figures 6, '7 and 8 are longitudinal sectional views of three different types of coaxial cable constructions builtup with the membrane or membranes of this invention, Figures 6a, 7a, 8a being end views thereof.

Figures 9, 10 and 11 are views in elevation illustrating the applicationof the helical membrane to multiconductor cables, Figures 9a, 10a, and llabeing end views thereof.

Figure 12 is apart sectional elevation'illustrating a device for relieving the stressin an extended helical membrane.

Referring more particularly to Figure 1' a layer of insulation I is applied'by extrusion or other known; means toa central support-2', which*may consist of wire or rod. The size ofthe'support will of course be varied to suit requirements-but it is substantially of the same diameter as thecentral conductor or core to which thehelicalmembrane is subsequently to'be applied. The insula- 'tion may consist of polythene, polytet-rafiuoroethylene, polystyrene or othersuitable dielectric. The insulating cover may be made to conform acourately to the desired dimensions by known methods. Thus the inner diameter as represented by .7: may be made to conform closely to that of the support by cooling the core so formed slowly as it issues from the extruder, or'by the ap plication-of external pressure as it cools.""lhe outerdiameter as represented by-y'may beadjusted to any desired accuracy by shaving itwith a cutting tool in a subsequent operation. 'Thezinsulatedcore is then given a fine helicalout 3 of very short pitch through the insulating layeras far as the support. At the same time it is parted from the support 2 across the narrow section where it may be adhering. The helical thread i so" formed is now unwound from the support by rotatingthe free end at a predetermined rate around'the latter, in such a manner that it relayer of insulation 6 applied by extrusion in the form of a sleeve which acts to lock the inner conductorand its surrounding thread or membrane 4 into fixed relationship with one another. An

. outer conductor 1 .is then applied over the insulation sleeved. Aprotective sheath 8 i applied over the conductor 7 to protect the conductor from corrosion and-moisture.

.The outer conductor may be of braided conducting wires, metal tape either helically or longitudinally applied, a lead sheath or some other desired form of semi-flexible conducting cylinder.

It will be understood that the cable may be comprised of several layers of helical membrane, separated by extruded longitudinal.orlapped corerings in the case where the diameter ratioof outer to inner conductor is very large. lThe direction of lay may be reversed with successive layers, in which case the separators. may :be omitted.

Thus as shownin Figures '7, Fla which illustrate a coaxialcable constructedfin:accordance with the invention, two'helices, an innerxhelixQand an outer helix it, are wrappedxaround acentral conductor H, being separatedbyian extruded insulating covering 12,.and encloseclbyanrextruded outer conductor .ltiwhi'ch may'beza lead sheath.

Referring nowto Figureslfi, 8a; .thereis shown a central conductor H ,:provided' with ;.a'.ilayer. of insulation M, over'which the helical membrane 4 is applied, an outer-conductor I5,1consisting of a layer of tape folded longitudinally;beingrdirect lysupported by .the membrane. 17- is'a waterproof covering. This form of thezinvention is particularly applicable to cable constructions de-.- signed to have a low. capacitance,:necessitating a central conductor. ofsmall diameter. :Byxuse of an inner insulation layer M, the inherentweakness of the conductor is. overcome andza better looking efiect is secured between the membrane and the central core by reason ofthe increased diameter.

Similarly thev helical, membrane construction may be used in:conjuncticn-zwithzother.:kncwn forms of cable-insulation.

(a) The inner conductor may first be-spiralled with a'circular thread of suitable insulating material and then the'helical membrane may be-applied thereon. I

(b) The helical membraneimay-be appliedover a braiding of insulating thread which "surround the central conductor.

(0) The helical membrane may beapplied over or under a disc-spaced construction.

The helical membrane may. also? be-applied to the construction. of multi-conductor. cables, two ormore slots being out. in the peripheryofthe core illustrated inFigurel prior lt oeelongating the membrane. These slots mayrber cut-longitudinally or helically. :The conductorsiare laid in the slots and locked in position by :means; of an insulating thread or by means. ofa-- second helical membrane. The helical membranesupporting the conductors may :conveniently .be wound on acentral insulating-rod v or. tube so; as to provide a ooherentrstructurezon whichtozassemble the conductors and to define precisely the lay of the helix.

Referring now to Figures 9, 9a. the cable is built up on a central tube l9'of insulating material carrying a helical membrane I8 provided with deep slots to locate the conductors 2| which are locked in position by a helically wrapped thread 22.

Referring to the construction illustrated in Figures 10 and 100. the central tube l9 supports a helix I-8 which is similarly slotted to receive the conductors 2|, the latter being locked in position by a second helix 23 which is of the same external diameter as theslotted helix I8 but whose inner diameter is of a size to; hold; the conductors in the bottom of the slots.

In the somewhat similar construction illustrated in Figures 11, 11a, a helical membrane 25 having shallow slots 26 is supported by the insulating tube |9. The conductors 2| are locked in position by a second and outer helix 29.

It will be appreciated that while no electromagnetic screen is shown in the several constructions illustrated in Figures 9 to 11, such a screen may be used, in which case it could. be supported directly or indirectly by the helical membranes.

The extension of the thread from its position when out as shown in Figure 1 to its final form in Figure 2 sets up some strain in the plastic material. Provided the ratio of the outer to inner diameter of the thread is low and the ratio of the final pitch of the membrane thread to its thickness is not excessive, this strain can be accommodated by a distortion of the membrane formed by the thread. Where both of these ratios are high, the strain involved may be excessive and it may be necessary to carry out a further forming process. In such a case the thread is extended approximately to its final pitch and is then passed between hot rollers which raise the temperature of the material to its softening point but well below its melting point. The temperature is not critical because in all practical cases the required strain should not exceed about 10 to 15%.

Referring now to Figure 12 there is illustrated a form of heating device through which an expanded helix is passed in order to relieve the stress set up by the expansion operation. The device comprises essentially a revolving tube 3| mounted to rotate in bearings 31 and incorporating a worm and worm wheel drive 33. Located centrally within the tube 3| are revolving conical rollers 35, between which the helical membrane 33 is fed. Rollers 35 are fast on spindles 39 journalled in bearings 36 in the tube, which is formed with an increased wall thickness or shouldered portion 40 for this purpose and to provide a circumferential bearing 4| for a worm annulus 42 meshing with a worm 43. The annulus 42 is formed on its opposite faces with rack teeth 44 which are in driving engagement with bevel pinions 45 on the roller spindles 3%. Located on opposite sides of the rollers 35 are slip rings 46, 41 for the introduction through corresponding apertures 48, 49 of hot and cold air respectively for heating and cooling the membrane. In operation the membrane 30 is fed in the direction of the arrows through the tube interior, being guided by a series of rollers 50 located downstream and upstream of the driven rollers 35, rollers 50 being freely mounted on studs 5| and carried by the tube 3|.

It will be understood that cables having a wide range of electrical and mechanical characteristics can be designed by using a membrane construction in accordance with the present invention. In general the effective permittivity can, for a given robustness, be made lower than by the use of conventional constructions. Thus a cable of 0.75" diameter inside the outer conductor formed with a polythene membrane of 15 mils thickness having a pitch of 0.2 ha an effective permittivity of approximately 1.10.

WhatIclaim is:

l. A method for the manufacture of insulation material for use in an electric cable including inner and outer conductors and an interposed helix of solid insulation; said method comprising extruding upon a former of predetermined size substantially the same as the size of the inner conductor to which the material is to be applied a layer of dielectric material of sufficient inherent rigidity to provide, when formed as a short pitch helix and subsequently extended, effective mechanical connection of the inner and outer conductors, helically cutting the dielectric material of said layer into a short pitch helix of solid insulation having a prescribed internal dimension corresponding to the external dimension of the inner conductor, and extending said short pitch helix.

2. A method for the manufacture of insulation material for use in an electric cable including inner and outer conductors and an interposed helix of solid insulation; said method comprising extruding upon a former of predetermined size substantially the same as the size of the inner conductor to which the material is to be applied a layer of dielectric material of sufficient inherent rigidity to provide, when formed as a short pitch helix having convolutions of substantially greater radial dimension from inner to outer diameter than axial thickness, effective mechanical connection of the inner and outer conductors,

helically cutting the dielectric material of said layer into a short pitch helix having convolutions of substantially greater radial dimension from inner to outer diameter than axial thickness, and separating the helix so formed from the former to provide a continuous helix of solid insulation having a prescribed internal dimension corresponding to the external dimension of the inner conductor.

3. A method for the manufacture of insulation material for use in an electric cable including inner and outer conductors and an interposed helix of solid insulation; said method comprising extruding upon a former of predetermined size substantially the same as the size of the inner conductor to which the material is to be applied a layer of dielectric material of such inherent rigidity that when out helically into a short pitch helix and separated from the former the helix will have an internal dimension substantially constant throughout the length of the helix so as to enable said helix to provide effective mechanical connection between the inner and outer conductors, and helically cutting the dielectric V material of said layer into a short pitchhelix.

4. A method for the manufacture of insulation material for use in an electric cable including inner and outer conductors and an interposed helix of solid insulation; said method comprising extruding upon a former of predetermined size substantially the same as the size of the inner conductor to which the material is to be applied a layer of dielectric material of sufficient inherent rigidity to provide, when formed as a helix having convolutions of substantially greater radial d1- mensinn Imm inner to guter. diameter than ,ex iel thickness. emotive smechanicaiconnection of the inner and outerconduc o s; helically cutting. th dielectric material of said layer intoa Short pitch helix having ,convolutians of substantially reater radial. dimension from inner to outer diamet r then-axial thickness and leaving said helix on and in cpnia ct with said former during the ,cutting of successive convolujzions; and separatmg the helix so formed from the former to provide .aucontinuou s. helix of solid insulatigm having a pnescribedu-internail dimension corresponding ,to Zthe vexterneldimension, of the inner conducter.

RAYMOND CHARLES MILDNER.

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